One of information output apparatuses in a word processor, personal computer, facsimile apparatus, and the like is an image printing apparatus which prints desired information such as a character or image on a sheet-like printing medium such as a paper sheet or film.
Various methods are known as the printing method of the image printing apparatus. In recent years, an ink-jet method has particularly received a great deal of attention because the ink-jet method enables noncontact printing on a printing medium such as a paper sheet, easily achieves color printing, and generates little noise. In terms of low cost and easy downsizing, the printer generally widely adopts a serial printing arrangement in which a printhead for discharging ink in accordance with desired printing information is mounted, and printing is done while the printhead is reciprocally scanned in a direction perpendicular to the feed direction of a printing medium such as a paper sheet.
FIG. 12 shows a conventional heater board 1100 of a printhead which prints by bubbling and discharging ink by using heat energy.
The conventional heater board (printing element board) 1100 comprises, on a single semiconductor substrate, heater resistors 1101 serving as electrothermal transducers, high-breakdown-voltage MOS transistors 1102 which switch a current, and bit selection circuits 1103 which select desired printing pixels (bits).
FIG. 13 shows an example of the layout of the heater resistors 1101 and high-breakdown-voltage MOS transistors 1102 on the conventional heater board 1100 of the printhead.
Heater resistors 1101a1 to 1101ax, 1101b1 to 1101bx, . . . , 1101m1 to 1101mx are connected to corresponding high-breakdown-voltage MOS transistors 1102a1 to 1102ax, 1102b1 to 1102bx, . . . , 1102m1 to 1102mx. 
In order to shorten the connection line between each heater resistor and a corresponding high-breakdown-voltage MOS transistor and effectively utilize the board area, the heater pitch as the heater resistor interval and the pitch of the high-breakdown-voltage MOS transistor which drives the heater are designed equal to each other.
Driving of the heater resistor has conventionally used a bipolar transistor. To cope with high density of heater resistors and low cost, the above-mentioned high-breakdown-voltage MOS transistor is being used.
For high-speed printing, it is desirable to simultaneously drive nozzles (heater resistors) as many as possible. However, a simultaneously supplied current is restricted because of the limitation on the current supply ability of the power supply and a voltage drop by the resistance of wiring from the power supply to a heater resistor.
For this reason, a plurality of heater resistors are driven by time division to discharge ink. For example, heater resistors are classified into a plurality of groups, and driven by time division so as not to simultaneously drive two or more heater resistors with in a group. This suppresses the total heater current, eliminating the need for supplying a large current at once.
FIG. 14 shows a heater resistor driving circuit for discharging ink from each nozzle.
Reference numeral 1101 denotes each heater resistor; 1102, each high-breakdown-voltage MOS transistor; 1104, a power supply line which is connected to the power supply; and 1105, each control terminal which is connected to a controller.
As shown in FIG. 14, the heater resistors 1101 and corresponding high-breakdown-voltage MOS transistors 1102 are classified into groups a to m in equal numbers.
More specifically, in group a, the power supply line 1104 is commonly connected to the heater resistors 1101a1 to 1101ax. The high-breakdown-voltage MOS transistors 1102a1 to 1102ax are series-connected to the corresponding heater resistors 1101a1 to 1101ax between the power supply 1104 and ground.
When the bit selection circuit 1103 outputs control signals 1106a1 to 1106ax to the heater resistors 1101 via the control terminals 1105, the switching circuits of the high-breakdown-voltage MOS transistors 1102a1 to 1102ax are turned on to supply a current from the power supply via the power supply line 1104 and heat the heater resistors 1101a1 to 1101ax. 
The arrangements of groups b to m are also the same as that of group a.
The control signals 1106a1 to 1106ax from the bit selection circuit 1103 are input to the control terminals 1105 to control driving of the corresponding high-breakdown-voltage MOS transistors 1102a1 to 1102ax. Since the heater resistors 1101a1 to 1101ax receive a voltage of 5 V or more, e.g., 16 to 24 V, the high-breakdown-voltage MOS transistors 1102a1 to 1102ax have a higher breakdown voltage than that of a general MOS transistor.
FIG. 15 is a timing chart showing the heater driving circuit in FIG. 14, i.e., a heater driving circuit for driving heater resistors belonging to each group.
Group a in FIG. 14 will be exemplified. The control signals 1106a1 to 1106ax are timing signals for driving the first to xth heater resistors 1101a1 to 1101ax belonging to group a. That is, the control signal 1106 represents a waveform input to the control terminal 1105 of each high-breakdown-voltage MOS transistor 1102 in group a. The high-breakdown-voltage MOS transistor 1102 is turned on (connected) for Hi and off (disconnected) for Lo. The remaining groups b to m operate similarly to group a.
In this manner, heaters in each group are sequentially driven by time division. The current in each group can always be controlled to a current of 1 bit (pixel printed by one nozzle) or less, and no large current need be supplied to heater resistors at once. FIGS. 16A and 16B show the sectional structures of a high-breakdown-voltage MOS transistor and normal-breakdown-voltage MOS transistor.
FIG. 16B shows a normal-breakdown-voltage NMOS transistor formed on a P-type semiconductor substrate. N+ diffusion layers 111 and 113 respectively form a source and drain, and a gate 112 is arranged between them.
FIG. 16A shows a high-breakdown-voltage NMOS transistor formed on a P-type semiconductor substrate. N+ diffusion layers 111 and 113 of the high-breakdown-voltage MOS transistor respectively form a source and drain, and a gate 112 is arranged between them, similar to the normal-breakdown-voltage NMOS transistor.
In the high-breakdown-voltage MOS transistor, the gate length is larger than that in the normal MOS transistor, and an N− diffusion layer 114 for maintaining a uniform field is arranged between the gate 112 and the drain 113, which yields a high breakdown voltage.
In recent years, higher-speed, higher-resolution printers are required, and the printhead of the printer is equipped with many nozzles at a high density. As for the arrangement of a heater board used for the printhead, it is necessary to increase the number of heaters (heater resistors) and decrease the pitch of heaters (heater resistors).
The heater board is constituted by forming a heater and driving circuit on a single semiconductor substrate. The number of heater boards formed from one wafer must be increased to reduce the cost. For this purpose, the heater board must be downsized.
However, an increase in heater density and downsizing of the heater board pose the following problems.
When the heater density is increased, the pitch of heater driving transistors is determined, and the unit area of the heater driving transistor decreases. As a result, the ON resistance of the transistor in driving the heater increases.
Also when the area of the driving circuit is decreased for downsizing the heater board, the transistor area decreases. The ON resistance of the transistor in driving the heater increases.
The heater and the transistor serving as a heater driving switch are series-connected to the power supply, as shown in FIG. 14. If the ON resistance of the transistor in driving the heater increases upon increasing the heater density or downsizing the heater board, power consumption of the transistor increase and the ratio of power consumption of the heater to application power decreases, resulting in low power use efficiency.
If heat generation increases in the transistor, generated heat is accumulated in the transistor to change the ink discharge characteristic, or destructs the printhead.
To prevent this, it is important to decrease the ratio of the ON resistance of the transistor in driving the heater to the heater resistance when increasing the heater density or downsizing the heater board.
As a method of decreasing the ratio of the ON resistance of the transistor in driving the heater to the heater resistance, the heater resistance value is increased to relatively decrease the ratio of the ON resistance.
In the use of the method of relatively decreasing the ratio of the ON resistance, if heating amount of heater is not changed, the voltage applied to the heater must be increased. Along with this, the power supply voltage rises.
That is, if the power supply voltage rises, the voltage applied to the high-breakdown-voltage MOS transistor for driving a heater also rises. The breakdown voltage of the high-breakdown-voltage MOS transistor must be further increased.
To increase the breakdown voltage of the high-breakdown-voltage MOS transistor, the gate length or the length of the drift region must be increased. In either measure, since the transistor area increases, it may be hard to downsize the heater board.
As described above, it is important to decrease the ON resistance of the transistor in driving the heater without increasing the transistor area when increasing the heater density or downsizing the heater board.